Strains of E. coli which have reduced amounts of 6-methyladenine in DNA (DNA adenine methylase, [dam] mutants) display multiple phenotypes. These include (i) an aberrant morphology, (ii) increased sensitivity to various carcinogenic and mutagenic agents, (iii) increased spontaneous mutagenesis, (iv) facilitated induction of lysogenic viruses, and (v) inviability in the presence of mutations in certain DNA repair genes. Isolation of the dam mutants has allowed the conclusion that adenine methylation has an important role in the post-replicational metabolism and repair of DNA. But because of mutants are only deficient and not clearly devoid of methylase, two important questions regarding the methylation of DNA adenine remain unanswered: (1) Is adenine methylation essential for cell viability? (2) Is dam the structural gene for the methylase or merely a gene which regulates methylase expression? This proposal seeks to answer these questions through a combined in vitro and in vivo approach exploiting the construction of dam delections and strains bearing conditional mutations in the methylase. Even if these approaches demonstrate that the dam product is not required for viability, they will strongly benefit research on methyladenine function in several other ways. In particular, they will allow the isolation of mutants devoid of DNA methyladenine; these should prove very useful as tools for probing the postulated role for methyladenine in mismatch repair. The studies of methyladenine function with the E. coli model system offers a new approach to the elucidation of the origin and maintenance of mutation, and thus, they may contribute significantly to a more thorough understanding of the process linking carcinogenesis and spontaneous congenital abnormalities in man.